String programmable nonvolatile memory with NOR architecture

ABSTRACT

A nonvolatile memory having a NOR architecture has a memory array including a plurality of memory cells arranged in rows and columns in NOR configuration, the memory cells arranged on a same column being connected to one of a plurality of bit lines; and a column decoder. The column decoder comprises a plurality of selection stages, each of which is connected to respective bit lines and receives first bit line addressing signals. The selection stages comprise word programming selectors controlled by the first bit line addressing signals and supplying a programming voltage to only one of the bit lines of each selection stage. Each selection stage moreover comprises a string programming selection circuit controlled by second bit line addressing signals thereby simultaneously supplying the programming voltage to a plurality of the bit lines of each selection stage.

TECHNICAL FIELD

The present invention refers to a string programmable nonvolatile memorywith NOR architecture.

BACKGROUND OF THE INVENTION

As known, many of the characteristics of a nonvolatile memory, such asreading and programming speeds, are determined by the architecture ofthe memory, as well as by the employed manufacturing process.

At present, extensively used nonvolatile memories have a so-called NANDand NOR architectures.

In NAND memories, groups of memory cells, arranged in series andbelonging to distinct memory words, are connected to respective bitlines so as to form strings of cells. One memory word is formed by aplurality of adjacent cells connected to different bit lines, while aword string is formed by a plurality of cell strings. In particular, aword string comprises a number of memory words equal to the number ofcells contained in a cell string.

NAND memories are advantageous mainly because they have a low bulk andallow the so-called string programming. In fact, all the cells in a cellstring can be programmed by biasing the gate terminal of each cell at aprogramming gate voltage, and the relevant bit line at a programmingdrain voltage. In this way, by the Fowler-Nordheim effect, controlledcharges reach a floating gate region of each selected cell.Consequently, in a single programming step, byte strings (normally 8 or16 bytes at a time) may be written. Programming of a NAND memory istherefore very fast.

On the other hand, NAND memories have very long access times andconsequently they do not meet the requisites of reading speed currentlydemanded in most applications. In addition, the manufacture of NANDmemories is complex and costly.

In NOR memories, instead, the memory cells belonging to a same columnare connected in parallel between a same bit line and ground. Inaddition, the memory cells belonging to a same memory word have theirgate terminals connected together via word lines and can be selectedsimultaneously for reading or for programming. Row and column decodercircuits allow a memory word to be addressed by selecting a word lineand a plurality of bit lines.

As compared to NAND memories, NOR memories have the advantage of havingvery short access times (thus they are faster during reading), moreover,they can be manufactured using a simpler and less costly fabricationprocess. However, NOR memories are disadvantageous in that allow onlyone memory word to be programmed at a time. Thus, it is not possible toperform string programming, and it is necessary to repeat theprogramming step entirely for each word to be stored in memory.Consequently, NOR memories are slow during programming and are notsuited for use in fields in which fast acquisition of a high amount ofdata is required (as in the case, for instance, of digitalphotocameras).

SUMMARY OF THE INVENTION

According to the present invention, a string programmable memory withNOR architecture is provided.

A memory having a NOR architecture has a memory array including aplurality of memory cells arranged in rows and columns in NORconfiguration, the memory cells arranged on a same row being connectedto one of a plurality of word lines; and the memory cells arranged on asame column being connected to one of a plurality of bit lines, and acolumn decoder.

The column decoder comprises a plurality of selection stages, each ofwhich is connected to respective bit lines and receives first bit lineaddressing signals. The selection stages comprise word programmingselectors controlled by the first bit line addressing signals andsupplying a programming voltage to only one of the bit lines of eachselection stage. Each selection stage moreover comprises a stringprogramming selection circuit controlled by second bit line addressingsignals, thereby simultaneously supplying the programming voltage to aplurality of the bit lines of each selection stage.

The string programming selection circuit comprises a plurality Of bitregisters that are loaded, during successive clock cycles, with aplurality of data bits, which are then written, simultaneously, to aplurality of bit lines as previously described. Inasmuch as the loadingtime of a bit register is significantly faster than the programming timeof a memory cell, the time saved by programming a string of wordssimultaneously far exceeds the time spent loading the registers inpreparation for the string programming cycle.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, some embodiments thereofare now described, simply to provide non-limiting examples, withreference to the attached drawings, wherein:

FIG. 1 shows a NOR memory of known type;

FIG. 2 illustrates a simplified block diagram of a column decoderbelonging to the memory of FIG. 1, implementing a first embodiment ofthe present invention;

FIG. 3 illustrates a simplified circuit diagram of a block of the columndecoder of FIG. 2;

FIG. 4 shows a circuit diagram of a part of the block of FIG. 3 ingreater detail;

FIG. 5 illustrates a simplified block diagram of a column decoderbelonging to the memory of FIG. 1, implementing a second embodiment ofthe present invention;

FIG. 6 illustrates a simplified circuit diagram of a block of the columndecoder of FIG. 5;

FIGS. 7 and 8 illustrate circuit diagrams of parts of the block of FIG.6; and

FIG. 9 shows patterns of signals for the column decoder circuit of FIG.5.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a two level nonvolatile memory having NORarchitecture, in particular with 128 columns per bit. The memory 1comprises a memory array 2, formed by a plurality of memory cells 3arranged in rows and columns, a row decoder 5, of known type, and acolumn decoder 6 having a structure described in detail with referenceto the following figures. The gate terminals of the memory cells 3belonging to a same row are connected together via word lines 9connected to the row decoder 5. In addition, the memory cells 3belonging to a same column have their drain terminals connected each toa respective bit line 11.

With reference to FIG. 2, the column decoder 6 comprises a first leveldecoder stage 15, a second level decoder stage 16, and a plurality ofbit selection stages 17. In particular, the bit selection stages 17 areequal in number to the bits forming a memory word (normally eight orsixteen). The example discussed below has an eight word memoryconfiguration.

The first level decoder stage 15 has inputs connected to an address bus20 supplying a group of four first level address bits ADD₆, ADD₃ and anoutput connected to a first level bus 22 and supplying sixteen firstlevel signals YN₀, . . . , YN₁₅, each having a high logic value and alow logic value.

The second level decoder stage 16 has inputs connected to the addressbus supplying a group of three second level address bits ADD₂−ADD₀ andoutputs connected to a second level bus 23 and supplying eight secondlevel signals YM₀. . . , YM₇, defining first bit line addressingsignals.

Each of the bit selection stages 17 has a group of first level inputs,connected to the first level bus 22, and a group of second level inputs,connected to the second level bus 23. The bit selection stages 17 arealso directly connected to the address bus 20 supplying second leveladdress bits ADD₂−ADD₀. The bit selection stages 17 are connected to theoutside of the memory 1, to receive a string-programming signal SP, andto circuits inside the memory (known and not illustrated) supplyinginitialization pulses IN and synchronization pulses ATD.

In addition, the bit selection stages 17 are each connected torespective groups of bit lines II of the memory array 2 and to arespective data bus line 24. Each group of bit lines 11 comprises, forexample, 128 bit lines 11.

FIG. 3 shows a simplified circuit diagram of a bit selection stage 17,comprising a plurality of selection branches 25 and a local decoder 26defining string decoder means, In particular, the selection branches 25are equal in number to the second level signals YM₀, . . . , YM₇ (eight,in the case illustrated).

The local decoder 26 is connected to the address bus 20, so as toreceive at the input the second level address bits ADD₂−ADD₀, and haseight outputs, each of which is connected to a respective selectionbranch 25 and supplies a local addressing signal S₀, . . . , S₇ defininga second bit line addressing signal. In addition, the local decoder 26(not illustrated in detail) comprises a combinatory circuit, obvious forthe person skilled in the art, for selectively setting one of the localaddressing signals S₀, . . . , S₇ at the low logic level, according tothe second level address bits ADD₂−ADD₀.

Each of the selection branches 25 comprises a first level selector stage27, a second level selector stage 28 (defining word programmingselection means), a string selector 29, and a local enable register 30(together defining string programming selection means).

Each first level selector stage 27 comprises a plurality of first levelselectors 31 (sixteen, in the case illustrated), preferably formed byNMOS transistors having drain terminals connected to respective bitlines 11 and source terminals connected to a respective first connectionline 34. In each first level selector stage 27, moreover, each firstlevel selector 31 has its gate terminal connected to a respective line22 ₀, 22 ₁, . . . , 22 ₁₅ of the first level bus 22 (not shown), toreceive one of the first level signals YN₀, . . . , YN₁₅, defining athird bit line addressing signal.

In each selection branch 25, the second level selector 28 and the stringselector 29, preferably formed by NMOS transistors, are connected inseries between the respective selector stage 27 and a second connectionline 35. In addition, each second level selector 28 has its gateterminal 28 a connected to a respective line of the second level bus 23(not shown), so as to receive one of the second level signals YM₀, . . ., YM₇, while each string selector 29 has its gate terminal 29 aconnected to an output of a respective local enable register 30(defining enable means).

The second connection line 35 is connected to a column amplifier 60(“sense amplifier”) and to a programming voltage source 70 supplying aprogramming voltage V_(PD). The output of the column amplifier 60 isconnected to a data line 24 _(j) of the data bus 24.

Each local enable register 30, described in detail hereinafter, receivesthe string programming signal SP, the sync pulses ATD and initializationpulses IN, and is connected to a respective output of the local decodercircuit 26, so as to receive one of the local addressing signals S₀, . .. , S₇. In addition, all the local enable registers 30 belonging to asame bit selection stage 17 have further respective inputs connected tothe data line 24 _(j) of the data bus 24. Each of the local enablecircuits 30 supplies on its own output a string enable signal YML₀, . .. , YML₇ to control the respective string selector 29 on and off.

With reference to FIG. 4, each local enable register 30 comprises a NORgate 40, an OR gate 41, and a bistable circuit 42 of the set-reset type.

The NOR gate 40 has a first input connected to the data line 24 _(j) ofthe data bus 24 and a second input connected to a respective output ofthe local decoder circuit 26 supplying one of the local addressingsignals S₀, . . . , S₇, here indicated for reasons of simplicity byS_(k). In addition, an output of the NOR gate 40 is connected to a setterminal 42 a of the bistable circuit 42 via a switch 43 which has acontrol terminal 43 a receiving the sync pulses ATD. Conveniently, theswitch 43 comprises a CMOS pass gate 45 having a first and a secondcontrol terminals both connected to the control terminal 43 a, theformer directly, and the latter via an inverter 46. Consequently, theswitch 43 is normally open and closes only in the presence of a syncpulse ATD.

The bistable circuit 42 preferably comprises a pair of inverters 48,arranged back-to-back and connected between respective drain terminalsof a first and a second drive transistors 49, 50, which have groundedsource terminals. The gate terminal of the first drive transistor 49forms the set terminal 42 a, while the gate terminal and the drainterminal of the second drive transistor 50 form, respectively, a resetterminal 42 b receiving the initialization pulses IN, and an outputterminal 42 c of the bistable circuit 42. Consequently, the outputterminal 42 c supplies a selection value (having a high logic value) ifa pulse is supplied to the set terminal 42 a, and a deselection value(having a low logic value) if a pulse is supplied to the reset terminal42 b.

The OR gate has a first input connected to the output terminal 42 c ofthe bistable circuit 42, a second input receiving the string programmingsignal SP, and an output connected to the gate terminal 29 a of therespective string selector 29 and supplying one of the string enablesignals YML₀, . . . , YML₇, here indicated for reasons of convenience byYML_(k).

The operation of the memory 1 is the following.

Before single word programming or string programming, the memory I iserased to bring the memory cells 3 into a non-programmed state, normallyassociated with the high logic value. A programmed state, instead, isassociated to the low logic value. Consequently, after erasing, it isnecessary to program only the memory cells 3 in which a low logic valueis to be stored.

During single word reading or programming, the string programming signalSP is brought to the non-active value (associated with the high logicvalue) to force the string enable signals YML₀, . . . , YML₇ to the highlogic value (FIG. 4). All the string selectors 29 are thus closed, andthe column decoder 6 operates in a traditional way. In particular, thefirst level decoder 15 sets one of the first level signals YN₀, . . . ,YN₁₅ at the high logic level, and all the other first level signals YN₀,. . . , YN₁₅ at the low logic level (FIG. 3). In this way, in each firstlevel selector stage 27, only one first level selector 31 is closed, andthe relevant bit line 11 is connected to its respective connection line34. Likewise, the second level decoder 16 sets one of the second levelsignals YM₀, . . . , YM₇ at the high logic value so as to cause therelevant second level selector 28 to close, while all the other secondlevel signals YM₀, . . . , YM₇ are kept at the low logic value.Consequently, only one bit line 11 is selected for reading orprogramming and is connected, via the column amplifier 60, to the databus 24 or to the programming voltage source 70.

When, instead, string programming is carried out, all the second levelsignals YM₀, . . . , YM₇ are brought to the high logic value, so as toclose all the second level selectors 28. The string programming signalSP is brought to the active value (associated with the low logic value)and alternatively allows the selection or the deselection value presenton the output terminals 42 c of the respective bistable circuit 42 to besupplied to the gate terminals 29 a of the string selectors 29(Figure4).

With reference to FIG. 4, an initialization pulse IN is first suppliedto the reset terminals 42 b of the bistable circuits 42, so as to bringall the output terminals 42 c of the bistable circuits 42 and the stringenable signals YML₀, . . . , YML₇ to the deselection value (in practice,all the bistable circuits 42 are brought in a reset state).

Then, in successive clock cycles, a control unit (of known type and notillustrated) supplies the data bus 24 with words having consecutivecolumn addresses and forming a string to be stored in the memory 1. Inthe described embodiment, one string is formed by eight words, forexample, of sixteen bits each.

The words of the string to be stored are sequentially and temporarilyloaded in the local enable registers 30 (FIG. 3).

At each clock cycle, the local decoder 26 sets in sequence one of thelocal addressing signals S₀, . . . , S₇ at the low logic value, whileall the other signals are kept at the high logic value.

When the local addressing signal S_(k) is brought to the high logicvalue, the low logic value is forced on the output of the NOR gate 40and on the set terminal 42 a, regardless of the logic value present onthe data line 24 _(j) of the data bus 24. Consequently, the bistablecircuit 42 does not switch and remains in the state previously set.

When the local addressing signal S_(k) is set at the low logic value,the output of the NOR gate 40 supplies a complementary logic value withrespect to the logic value present on the data bus 24. Thiscomplementary logic value is transferred to the set terminal 42 a of thebistable circuit 42 upon arrival of a sync pulse ATD, when the switch 43is closed. Consequently, if the data line 24 _(j) of the data bus 24supplies the high logic level, the low logic level is supplied to theset terminal 42 a, and the bistable circuit 42 remains in the initialreset state. If, instead, a low logic level is present on the data line24 _(j) of the data bus 24, the high logic level is supplied to the setterminal 42 a, the bistable circuit 42 switches to a set state, and theoutput terminal 42 c is brought to the selection value.

In practice, at the end of loading of the words of the string to bestored, the bistable circuits 42 associated to memory cells 3 to beprogrammed at the low logic value are in the set state, and all theothers are in the reset state.

Since, as has been mentioned previously, the string programming signalhas the active value (low logic value), the values present on the outputterminals 42 c of the bistable circuits 42 are transferred onto theoutputs of the respective OR gates 41.

Consequently, the string enable signals YML₀, . . . , YML₇ associatedwith bistable circuits 42 in the set state are at the high logic value,and their respective string selectors 29 are closed. The string enablesignals YML₀, . . . , YML₇ associated with bistable circuits 42 in thereset state are at the low logic value, and their respective stringselectors 29 are open.

In this way (FIG. 3), only the bit lines 11 connected to memory cells 3that are to be programmed within the word string to be stored areselected.

The selected memory cells 3 may be then written and verified as usually,until the desired programming level is reached.

The time for programming a plurality of words forming a string istherefore equal to the time required for programming the slowest memorycell 3 among the selected ones, and is therefore considerably shorterthan in known memories, where the time for programming a plurality ofwords is proportional to the number of the words.

FIGS. 5-8, wherein the parts in common with FIGS. 1-4 are designated bythe same reference numbers, illustrate a second embodiment of thepresent invention, which, in particular, may be used for multilevelnonvolatile memories. For convenience, hereinafter reference will bemade to a four-level memory, without thereby limiting thereto.

FIG. 5 shows a column decoder 106 differing from the column decoder 6 ofFIG. 2 in that the bit selection stages 17 are replaced by columnselection stages 55. In particular, as for the bit selection stages 17of FIG. 2, the column selection stages 55 are connected to the firstlevel and second level decoders 15, 16, namely, to the first leveldecoders 15 through the first level bus 22, supplying the first levelsignals YN₀, . . . , YN₁₅, and to the second level decoders 16 throughthe second level bus 23, supplying the second level signals YM₀, . . . ,YM₇. Each column selection stage 55 is moreover connected to a pluralityof bit lines 11 (for example, 128 lines), to the address bus 20, and toa respective pair of data lines 24 a, 24 b of the data bus 24, andreceives the string programming signal SP, the sync pulses ATD, and theinitialization pulses IN.

With reference to FIG. 6, a column selection stage 55 comprises aplurality of selection branches 56 (eight, in the case in point) and thelocal decoder 26.

As in FIG. 3, the local decoder 26 is connected to the address bus 20,so as to receive the second level address bits ADD₂−ADD₀, and has eightoutputs, each connected to a respective selection branch 56 andsupplying a respective local addressing signal S₀, . . . , S₇.

Each selection branch 56 comprises a first level selector stage 27,formed by a plurality of first level selectors 31, a second levelselector 28, and a string selector 29. The source terminals 29b of thestring selector 29 are connected together and to the data bus 24,through the column amplifier 60, as well as to the programming voltagesource 70. In addition, each of the selection branches 56 comprises adata register 61, a comparator 62, and a local enable register 63,which, all together, define enable means.

The data registers 61 are each connected to a respective output of thelocal decoder 26, and receive a respective local addressing signal S₀, .. . , S₇, and to the respective pair of data lines 24 a, 24 b of thedata bus 24. In addition, each data register 61 has two outputsconnected to first inputs 62 a of a respective comparator 62, which hastwo second inputs 62 b connected to the column amplifier 60.Furthermore, each of the comparators 62 has an output 62 c connected toa respective local enable register 63 and issuing a respectiveprogramming interrupt signal PI₀, . . . , PI₇. Each comparator 62 isformed by a combinatory circuit (for example, formed by XOR gates 67 andone AND gate 68—FIG. 8) thereby allowing each first input 62 a to becompared with a respective second input 62 b of the comparator 62. Whenthe logic values present on the first and second inputs 62 a, 62 b ofthe comparator 62 are equal two by two, the respective programminginterrupt signal PI₀, . . . , PI₇ is set at a high logic value;otherwise, it is set at a low logic value.

The local enable registers 63 are connected to a respective output ofthe local decoder 26, supplying each local enable register 63 with alocal addressing signal S₀, . . . , S₇, and to the data lines 24 a, 24 bof the data bus 24. In addition, the local enable registers 63 receivethe string programming signal SP, the sync pulses ATD, and theinitialization pulses IN, and have outputs connected to respective gateterminals 29 a of the string selectors 29.

With reference to FIG. 7, a local enable register 63 comprises the sameelements as the local enable register 30 of FIG. 4. The local enableregister 63 moreover comprises an AND gate 71 having inputs connected tothe pair of data lines 24 a, 24 b of the data bus 24, and an outputconnected to the first input of the NOR gate 40. Furthermore, in thelocal enable register 63 the bistable circuit 42 has a first reset input42 b and a second reset input 42 d. In particular, the first resetterminal 42 b receives the initialization pulses IN, and the secondreset terminal 42 d is connected to the output 62 c of a respectivecomparator 62 and receives a respective programming interrupt signal,here indicated PI_(k). The second reset terminal 42 d is connected to agate terminal of a third drive transistor 73, of NMOS type, which hasgrounded source terminal and drain terminal connected to the output 42 cof the bistable circuit 42. In practice, the second and third drivetransistors 50, 73 define an inverted logical sum gate.

The operation of the column selection stage 55 will now be describedwith reference to FIGS. 6-8.

When string programming is performed, one of the first level signalsYN₀, . . . , YN₁₅ is set to a high logic level, so as to select, in eachfirst level selector stage 27, a respective bit line 11. In addition,the second-level signals YM₀, . . . , YM₇ are set at a high logic value,the string programming signal SP is brought to the active value (highlogic value), and an initialization pulse IN is supplied, so as to bringall the bistable circuits 42 in the reset state, as evident from FIG. 7.

Then, in successive clock cycles, words forming a string to be storedare loaded in the data register 61. FIG. 9 illustrates the patterns ofthe signals present on the data bus 24 and on the address bus 20 duringloading of an eight-word string. In particular, two bits are loaded ineach data register 61 (each memory cell may store four possibleprogramming levels).

At the same time, the bistable circuits 42 are brought in the set stateif connected to a memory cell 3 to be written; otherwise, they are keptin the reset state. In fact, for four-level memory cells thenon-programmed state normally corresponds to the value “11” (two bits atthe high logic value). Consequently, when the memory cell 3 is to remainin the non-programmed state, both the data lines 24 a, 24 b of the databus 24 are at a high logic value. The output of the AND gate 71 is at ahigh logic value, and, when the local addressing signal S_(k) is set atthe low logic value and a sync pulse ATD is supplied, the low logicvalue is supplied to the set terminal 42 a of the bistable circuit 42.Consequently, the bistable circuit 42 is kept in the reset state, andthe string enable signal YML₀, . . . , YML₇ is at the low logic valueand determines opening of the respective string selector 29 (FIG. 6).The corresponding bit line 11 and the memory cell 3 connected theretoare thus deselected.

The memory cell 3 must instead be written when the value to be stored isother than “11,” i.e., when at least one of the data lines 24 a, 24 b ofthe data bus 24 is at the low logic value. In this case (FIG. 7), a highlogic value is transferred to the set terminal 42 a through the AND gate71 and the NOR gate 40, and the bistable circuit 42 switches to the setstate and closes the respective string selector 29 (FIG. 6).

In this way, the memory cells 3 to be written are selected.

Next, a first writing step, to reach a first programming level of theselected memory cells 3 (corresponding to the value “10”), and a verifystep, in which the selected memory cells 3 are read sequentially, arecarried out. During verify, the column amplifier 60 supplies, to thesecond inputs 62 b of the comparators 62, pairs of logic valuescorresponding to the values stored in the memory cells 3.

The comparators 62 compare the values stored in the data registers 61with the values stored in the memory cells 3. These values are equal forthe memory cells 3 that have reached the desired programming level. Inthis case, the corresponding programming interrupt signals PI₀, . . . ,PI₇ are set at the high logic value, so as to bring the respectivebistable circuits 42 into the reset state and cause the respectivestring selectors 29 to open. The memory cells 3 that have reached thedesired programming level are thus deselected, and the others remainselected.

The process is then repeated until programming of the memory cells 3associated with the string to be stored is completed.

The advantages of the present invention emerge clearly from theforegoing description.

The described memory 1, in fact, can be string programmed even if it hasNOR architecture, and may be programmed considerably faster than knownNOR memories. Consequently, the memory I has a reduced access time, lowmanufacture costs, and high programming speed, and can thus beadvantageously used in a wide range of applications.

Finally, it is clear that numerous modifications and variations can bemade to the described memory, all falling within the scope of theinvention, as specified in the attached claims.

In particular, the number of bit selection stages or column selectionstages may be varied so as to obtain words of different length. Inaddition, the column decoder may comprise a different number ofselection branches, so enabling programming of strings of differentlength.

In addition, instead of using a local decoder 26 for each bit selectionstage 17 or for each column selection stage 55, it is possible to use asingle global string decoder arranged like the first and second leveldecoder stages 15, 16 and connected, via a respective bus, to the bitselection stage 17 or the column selection stage 55, even though thisentails a bigger consumption of area of the buses present inside thememory 1.

What is claimed is:
 1. A nonvolatile memory having NOR architecture,comprising: a memory array including a plurality of memory cellsarranged in rows and columns in NOR configuration; a plurality of bitlines; the memory cells arranged in a same column being connected to oneof the plurality of bit lines; a column decoder, said column decoderincluding: a programming voltage source stage; a decoder stagegenerating first bit line addressing signals; a plurality of selectionstages, each of which is connected to respective bit lines and receivessaid first bit line addressing signals, said selection stages eachhaving word programming selectors controlled by said first bit lineaddressing signals and connecting said programming voltage source stageto only one of said bit lines; a string decoding circuit generatingsecond bit line addressing signals, each said selection stage having astring programming selection circuit controlled by said second bit lineaddressing signals and simultaneously connecting said programmingvoltage source stage to a plurality of said respective bit lines.
 2. Thenonvolatile memory according to claim 1, wherein each selection stagecomprises a plurality of selection branches and in said word programmingselectors comprise a second level selector for each selection branch,each said second level selector being coupled between at least one bitline and said programming voltage source stage and being controlled by arespective one of said first bit line addressing signals, and in thatsaid string programming selection circuit comprises a string selectorcoupled between said second level selector and said programming voltagesource stage and is controlled by a respective string enable signalcorrelated to a respective second bit line addressing signal.
 3. Thenonvolatile memory according to claim 2, wherein each of said selectionbranches comprises a first level selector, coupled between a group ofbit lines, and a second level selector, said first level selectors beingcontrolled by third bit line addressing signals; each of said firstlevel selectors connecting a single bit line of said group of bit linesto a respective second level selector.
 4. The nonvolatile memoryaccording to claim 2, further including: a data bus, said stringprogramming selection circuit comprising enable means having a firstinput connected to said data bus; a second input receiving a respectivesecond bit line addressing signal; and an output connected to arespective control terminal of said string selectors and supplying arespective second bit line addressing signal, thereby alternativelyselecting and deselecting the bit lines connected to a respective secondlevel selector.
 5. The nonvolatile memory according to claim 4, whereinsaid enable means comprises: first memory means having a first controlterminal connected to said data bus; a second control terminal receivinginitialization pulses; and an output terminal connected to a respectiveone of said string selectors and supplying said string enable signal. 6.The nonvolatile memory according to claim 4 wherein said first memorymeans comprises a bistable circuit of the set-reset type.
 7. Thenonvolatile memory according to claim 5 wherein said enable meanscomprise a logic circuit and switching means, said logic circuit havingan input connected to said data bus and an output connected to a firstcontrol terminal of said first memory means, and said switching meansbeing connected between said logic circuit and said first controlterminal of said first memory means and having an own control terminalreceiving sync pulses.
 8. The nonvolatile memory according to claim 7,wherein said string decoding means comprises a local string decoder foreach selection stage, each said local string decoder having an inputconnected to an address bus and outputs connected to respective ones ofsaid logic circuits of said enable means and supplying said second bitline addressing signals.
 9. The nonvolatile memory according to claim 5wherein said enable means comprises second memory means having a firstinput connected to a respective output of said string decoding means anda second input connected to said data bus.
 10. The nonvolatile memoryaccording to claim 9, wherein said selection stage comprises columnamplifier means and in that said enable means further comprisecomparator means having a first input connected to said second memorymeans, a second input connected to said column amplifier means, and anoutput connected to a third control terminal of said first memory means,and supplying respective programming interrupt signals.
 11. Thenonvolatile memory according to claim 10, wherein said first memorymeans comprises a set-reset bistable circuit, and in that it comprises alogic sum circuit arranged between said second and third controlterminals of said first memory means and a reset input of said bistablecircuit.
 12. A method comprising, in a nonvolatile memory having NORarchitecture and comprising a memory array including a plurality ofmemory cells arranged in rows and columns in NOR configuration, thememory cells arranged in a same column being connected to one of aplurality of bit lines; and a column decoder including a plurality ofselection stages, each of which is connected to respective bit lines,the method comprising: generating a programming voltage; generatingfirst bit line addressing signals; selecting only one bit line for eachselection stage according to said first bit line addressing signals andsupplying said programming voltage to said only one bit line; generatingsecond bit line addressing signals; and selecting a plurality of bitlines for each selection stage according to said bit line addressingsignals; and simultaneously supplying a programming voltage to saidplurality of bit lines.